Crater cathode



Feb. 6, 1940. 5, EVANS 7 2,189,629

CRATER CATHODE Filed May 26, 1958 WI NESSES: INVENTOR .ZZ ZdW 7 George. S; Evans.

ATTORNE Patented Feb. 6, 1940 UNITED STATES PATENT OFFICE CRA'IER CATHODE George S. Evans, East Orange, N J., vassignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 26, 1938, Serial No. 210,185

Claims.

My invention relates to gaseous electric dis cathode structure in which a partially enclosed region is'provided about a thermionically emissive' cathode surface, this region being especially adapted to maintain a high degree of ionization in a gas or vapor within its confines.

Another object 'of my invention is to provide a cathode structure with means for constantly maintaining a higher pressure of a gas or vapor in the region of the cathode surface than in the vicinity of an anode and/or a control electrode within a gaseous discharge tube.

Still another object of my invention is to provide a cathode structure adapted to continually supply a stream of gas or vapor drawn from a reservoir of liquid material within an electrical discharge tube to the region about a thermionically emissive cathode surface.

Still another object of my invention is to provide an electrical discharge tube structure adapted to establish paths of circulation in which a liquid is vaporized, the vapor conducted to 'a restricted region about a thermionically emissive cathode surface, is then conducted away from this region, recondensed and returned to the aforesaid liquid.

Other objects of my invention will become apparent upon reviewing the following description taken in connection withthe drawing, in which;

Figure 1 is a view, partly in elevation and paruy in section, of one type of tube embodying the principles of my invention; and 1 Fig. 2 is a similar view of another type of tube embodying the principles of my invention.

My copending application Serial No. 210,184 discloses certain types of cathode structure which are adapted to provide a highly ionized body of vapor at a substantial pressure in an enclosure, the walls of which are heated to a ther mionically emissive temperature, .while at the of the cathode is formed of sheet metal which is sealed to the glass or other insulator which electrically separates the control electrode and anode of the tube from the cathode. By such an arrangement, the somewhat elaborate structure of the cathode chamber may be fabricated and installed in the metallic portion of the tube wall before the glass insulation is put in place.

Referring particularly to Fig. 1, the portion of the tube in the neighborhood of the cathode comprises a metallic cup I which may be of iron or any other metal having a suitably high melting point. If ordinary metals are employed for the cup I, the portions thereof to which glass insulators are intended to be sealed should be made of some metal or alloy known in the art to successfully form vacuum-tight seals with the glass or other insulation employed. For example, if

the art,'the alloy Kovar may be welded to the reinainder of the metallic cup I by conventional methods, or, alternatively, the entire cup I may itself consist of Kovar.

Within the metal cup I, 'I support a heating device 3 in the form of a hollow cylinder which may'consist of any suitable insulating refrac-- tory material. For example, the cylinder 3 may be molded from a mixture of barium and strontium oxides by methods well known in the art; alternatively it may be molded from zircon and thensintered. In good heat-conducting relation to the cylinder 3, preferably imbedded in its walls when it is molded, is a heater 4 which may be of nickel or of any other suitable resistance metal, such as the alloy Konal described in Patent No.

2,030,112 to E. F. Lowrey and assigned to the aforesaid company. The'heater wire 4 has one end extended downward to make contact with a liquid which partially fills the cup I and which will be. more fully described later, and the other end passes-through the wall of the metallic cup by an insulating seal made by methods well known in the tube art. For example, the seal may consist of an insulating collar 5 of the adjacent bore-silicate glass which is sealed to the alloy Kovar on the metal cup 1 about the periphery of the collar 5 and is sealed to a short section of the same alloy Kovar which is inserted in the lead from the heater 4 where it passes through the collar 5.

The cup I is provided with a re-entrant channel 6 which extends from the level of the partition 2 down to a point near the bottom of the cup I. It may be found desirable in some cases to provide the partition 2 with a number of perforations I. An inleading wire 9 for current to the tube may be welded to the cup I at any desired point thereon. When the tube is undergoing exhaust, there is introduced into it a suitable quantity of a conducting and readily vaporizable liquid 8 such as mercury and the level of this liquid is adjusted to such a height that it makes contact with the lower end of the heater 4 when the tube is in the position illustrated in the drawing.

When the cup I has been constructed in th above-described manner, the vacuum-tight enclosure may be completed by a wall II of any desired shape and material. As illustrated in Fig. l, the wall I I may suitably be formed of borosilicate G702P glass which may be sealed at I2 to the upper edge of the cup I. Alternatively it may be of ordinary German glass or of lead glass. Anode I3 and a control electrode I4, both of conventional form, may be sealed by well known means through the wall I I. In some cases, it may be desirable that the Wall II,'instead of being formed of glass or other insulating material, may be composed of the same metal as the cup I and, in fact, constitute a mere continuation thereof. In such a case, the inleading conductors to the anode I3 and control electrode I4 may be sealed vacuum tight through the container wall by providing suitable openings edged with Kovar in the latter, and sealing thereto collars of borosilicate or other glass very similar to the collar 5 already mentioned. The collars are sealed in turn to the inleading conductors. It is even'possible to assemble the entire tube structure with its glass insulation loosely placed in position and to start the evacuation of the tube prior to sealing of the glass insulation to the metallic portions, if so desired. The actual completion of the sealing operation can be carried out during the heating of the tube on exhaust, which is a normal step in the custom ary procedure of tube evacuation. On the other hand, it may be desirable to seal the glass insulation to the metallic portions completely before evacuation is started.

. Having assembled the tube structure above de scribed, it may be exhausted to a high degree of vacuum by methods well known in the art, the liquid 8 being introduced, and the entire device sealed off from the pumps.

When current is supplied to the heater 4 from any suitable source, the surface of the liquid 8 immediately below the central core of cylinder 3 will be heated, and the vapor of the liquid 8 will flow upward through the central opening of cylinder 3 to be recondensed upon the Walls of the container II. It will then flow down the latter until it reaches the partition 2 which will cause'it to flow into the side channel 6 and so back to the bottom of the cup I. By regulating the height of the liquid 8 so that it stands nearer to the bottom of the cylinder 3, the density and degree of ionization of the vapor adjacent the interior wall of the cylinder 3 will be increased. Similar results will follow from increasing the amount of current through the heater 4. An increase in current in the latter will, of course, likewise increase the thermionic emissivity of the internal wall of the cylinder 3. By suitably adjusting the height of the liquid 8 at the time the tube is exhausted and the size of the heater 4, and by suitably cooling the exterior of the container I I, the difference in vapor pressure between the interior of the cylinder 3 and the region of the container I I surrounding the anode I3 and the control electrode I 4 may be fixed at substantially any desired value.

Referring particularly to Fig. 2, the container I I, the anode I3, control electrode I4, the seals 5 and I2, and the heater 4 are similar to those already described for Fig. 1 and require no separate description. However, the structure of the oathode chamber differs from that of Fig. 1. This cathode chamber consists of a double-walled cylinder I5 which may be of some thin wall metal such as nickel. The lower portion of the inner cylinder I6 is of relatively restricted area, but it is provided with a shoulder I I near its upper portion so that it forms a cup within which may be positioned a collar of refractory insulating material I8 in which is imbedded the heater 4. The collar I8 may, for example, consist of a molded mixture of barium oxide and strontium oxide or it may consist of some other refractory insulation, but, if the latter, its interior surface is preferably coated with either a layer of mixed barium and strontium oxides superposed on a coating of nickel or other metal, or it may be coated with a layer of these materials mixed and sintered with particles of some metal such as nickel. The aforesaid layer is thus constructed so that it will be a free emitter of electrons when it is heated by the heater 4. The upper and lower ends of the cylinder I5 are closed by partitions, and the entire interior of the resulting structure is exhausted to a high degree of vacuum to render it as good a heat insulator as possible. For the latter reason, the metal composing its walls should be as thin as possible and should preferably have as low an electrical conductivity as possible. The cylinder I5 is supported upon and connected to the metal cup I by supports I9 which leave relatively restricted channels between the central cylinder I8 and the space surrounding the outer cylinder I5.

Alternatively, the metallic cylinders I5, I6, I! may be replaced by any other good heat insulator of substantialy similar surface contour, since its functions are largely those of heat insulating the interior of the collar I8 from the space surrounding the cylinder I5 and of providing restricted passages through which liquid may flow from said interior of collar I8 to said exterior of cylinder I5. When the portion I5, IE, IT is of metal, as described, the lower end of the heater 4 may be welded or otherwise electrically connected to the portion I I, and the heating current will then flow through the walls of the metallic cylinders and through supports It to inleading wire 9. -On the other hand, if some form of refractory insulator is substituted for the member I5, I6, II, it will be necessary to extend the lower end of the heater 4 sufficiently to connect it to the cup I.

When the aforesaid tube is exhausted, it is filled'to about the level indicated with a liquid 8 which may be mercury or some similar conducting liquid. The level of the mercury is made such that it rises slightly above the partition Il. When'the proper current is supplied to the heater 4, the liquid within the cylinder I5 will be rapidly vaporized and will flow out through the restricted opening 2| in the upper face thereof to be recondensed upon the walls of the tube I I. From there, it will flow down again to the region within the cup I external to the cylinder I5. There will accordingly be a continual slow flow of the liquid 8 through the openings in-"the supports I9 up through the central cylinder I6 in the heated cup formed by the refractory. cylinder I8 and the partition I'I.v The size of the channels through which this liquid flow takes place should be so proportioned that, when the heater 4 raisesthe interior/face of the collar I8 to an electron-emitting temperature, the desired pressure diiference will exist between the vapor inside the refractory collar I8 and the region surrounding the anode I3 and the control electrode I4. Proper cooling of the outside of the container I I will cause more rapid condensationof the emitted vapor and tend to increase the pressure difference between the regions just mentioned.

By making the diameter of the cylinder I6 and the distance between the lower end of cylinder I6 and the bottom of cup I smaller, the conduction of heat from the interior of cylinder I8 to the liquid 8 in the space surrounding cylinder I may be decreased to substantially any desired amount. It is accordingly possible to maintain substantially as high a vapor pressure inside the collar I8 as may be desired, while at the same time maintaining'a relatively cool liquid surface in the region surrounding the cylinder I 5, and as a consequence of the latter, maintaining a relatively low vapor pressure in all portions of the container II except the interior of the refractory collar I8.

To fully illustrate one specific embodiment of I my invention, the container II may have an outside diameter of approximately 6 inches and an over-all height of approximately 5 inches. The metallic cup I may be of sheet Kovar about .050 inch thick and have an external diameter of about 2% inches'and be about 2 /2 inches in height from its bottom to the seal I2. In Fig. 1, the refractory cylinder 3 may be about inch in outside diameter, inch in inside diameter and inch high. The heater wire 4 may be designed to carry about 11 amperes at about 5 terminal volts. The distance from the lower edge of cylinder 3 to the surface of the liquid 8 may be about inch, and the internal diameter of the v tube 6 may be about inch.

In Fig. 2, the collar I8 may have an external diameter of about% inch and its orifice 2I have an internal diameter of inch and be /2 inch high. Below said orifice the internal diameter of cylinder I8 may be /2 inch. The internal cylinder I6 may be inch in diameter and its lower end be inch above. the bottom of cylinder I. The cylinder I5 may be 1 /4 inches in diameter and 1 inches high. The anode I3 and its inleading wires and the lead 9 may be proportioned so that the average output current of the tube in each figure may be 4 amperes. The spacing between the lower end of the anode I3 and the upper face of the refractory collar I8 in Fig. 2, or 3 in Fig. 1, may be about 2 /2 inches. Such a tube will be adapted to operate on a line of about 110 terminal volts.

While I have described two specific embodia ments of my invention, the principles thereof are of broader application which will be evident to those skilled in the art. I accordingly desire that the following claims shall be restricted only insofar as is necessary in view of their express terms and of the limitations imposed by the prior art.

I claim as my invention:

1. In combination with a vacuum tight container comprising a cup-shaped metallic wall portion, an anode sealed thereto by vacuum-tight insulation and a vaporizable liquid in said cup, a cathode structure comprising a heating resistor, a surface-of thermionically-emissive material, in heat-receiving relation to said resistor and supported within said cup-shaped wall portion above said liquid; the-mouth of said cup facing in such a direction that the vapor formed when said liquid is heated by said surface is constrained to flow from the vicinity of said surface to the region within said container in the neighborhood of said anode, and walls forming a restricted channel through which the condensate from said vapor is constrained to flow back to the neighborhood of said cathode structure.

2. In combination with a' vacuurn tight container comprising a cup-shaped metallic wall portion, an anode sealed thereto by vacuum-tight insulation and a vaporizable liquid in said cup, a cathode structure, comprising a heating resistor, a surface of thermionically-emissive material in heat receiving relation to said resistor and sup- :ported within said cup-shaped wall portion above said liquid, the mouth of said cup facing in such a direction that the vapor formed when said liquid is heated by said surface is constrained to flow from the vicinity of said surface to the region tainer comprising a cup-shaped metallic wall portion, an anode sealed thereto by vacuumtight insulation and a vaporizable liquid in said cup, a cylindrical cathode structure comprising a heating resistor, a surface of thermionicallyemissive material in heat-receiving relation to said resistor and supported within said cupshaped wall portion above said liquid, an aperture throughthe mouth of said cup facing in such a direction that the vapor formed when said liquid is heated by said surface is constrained to flow from the vicinity of said surface to the region within said container in'theneighborhood of said anode, and walls forming a restricted channel through which the condensate from said vapor is constrained to flow back to the neighborhood of said cylindrical cathode structure.

4. In combination with a vacuum-tight container comprising a cup-shaped metallic wall portion, an anode sealed thereto by vacuumtight insulation and a vaporizable liquid in said cup, a cathode structure comprising a heatingresistor, a surface of thermionically-emmissive material in heat-receiving relation to said resistor and supported within said cup-shaped wall porchannel through which condensate of said vapor is constrained to flow back to said vaporizable liquid.

5. In combination with a vacuum-tight container comprising a cup-shaped metallic wall portion, an anode sealed thereto by vacuum-tight insulation and a vaporizable liquid in said cup, 2.

cylindrical. cathode structure comprising a heating resistor, asurface of thermionically-emissive material in heat-receiving relation to said resistor and supported within said cup-shaped wall por tion above said liquid, the mouth of said cup facing in such a direction that the vapor formed when said liquid is heated by said surface is constrained to flow from the vicinity of said surface to the region within said container in the neighborhood of said anode, and a heat-insulating body separating said cylindrical cathode structure from the walls of said container and forming a restricted channel through which condensate of said vapor is constrained to flow back to said vaporizable liquid.

GEORGE S. EVANS. 

